Conventionally, a structure of welding and joining ends of frame materials has widely been adopted for an assembly of a structural object with a hexahedral frame. However, the assembly of the structural object by welding leads to such problems that an assembly worker is limited to a license holder and that long time is required for repair work due to significant post-assembly deformation caused by welding. To handle these problems, a structure of adding a member that couples the frame materials to a corner of the hexahedral frame and fastening the frame materials by a bolt or a rivet to obtain required strength has been disclosed (for example, see PTL 1, 2). In addition, a structure of fastening the frame materials by the bolt or the rivet without using the member that couples the frame materials has been disclosed (for example, see PTL 3).